Mitosis Stage by Stage
~5 min read · Tier toggle changes depth (T1 plain · T2 standard · T3 rigour)
You should understand the basic phases of the cell cycle and recognise that mitosis is the division of the nucleus, distinct from cytokinesis, which divides the cytoplasm.
Why this matters
Mitosis ensures genetic fidelity in every somatic cell division.
Without the precise mechanical separation of chromosomes, multicellular organisms could not maintain tissue integrity or repair damage. This lecture dissects the five stages of mitosis, focusing on the structural changes in chromosomes and the spindle apparatus that drive accurate segregation. Understanding these stages is fundamental to diagnosing cancer, where mitotic control is often lost, and to appreciating the mechanical elegance of cellular division.
The Core Concept: Segregation without Loss
Imagine a library where every book must be copied exactly once, and then the two identical sets must be moved to two separate rooms without any pages being torn or lost. Mitosis is this process. It begins with the condensation of long, thin chromatin fibres into visible chromosomes. Each chromosome consists of two identical sister chromatids joined at a centromere. The cell then builds a structure called the mitotic spindle, made of microtubules, which acts as a scaffold. This spindle attaches to the chromosomes and physically pulls the sister chromatids apart to opposite poles of the cell. The key idea is that mitosis is a mechanical sorting machine: it does not create new genetic information, but it ensures that each daughter cell receives a complete and identical set of chromosomes.
Prophase and Prometaphase: Preparation and Attachment
Prophase marks the beginning of mitosis. The chromatin condenses into distinct chromosomes, becoming visible under a light microscope. The nucleolus disappears, and the centrosomes, which contain centrioles in animal cells, migrate to opposite poles of the cell. As the centrosomes move, they nucleate the formation of the mitotic spindle. In prometaphase, the nuclear envelope breaks down, allowing spindle microtubules to access the chromosomes. Kinetochores, protein complexes located at the centromere of each chromatid, begin to interact with the microtubules. This stage is critical for error checking; the cell ensures that kinetochores are attached to microtubules from opposite poles before proceeding.
Metaphase: Alignment and Tension
Metaphase is defined by the alignment of chromosomes at the metaphase plate, an imaginary plane equidistant from the two spindle poles. The spindle microtubules exert pulling forces on the kinetochores. For a chromosome to align stably, its sister chromatids must be attached to microtubules originating from opposite poles. This creates tension across the centromere. The cell has a checkpoint at this stage, known as the spindle assembly checkpoint. If any kinetochore is unattached or if the tension is incorrect, the cell arrests in metaphase. This prevents aneuploidy, a condition where daughter cells have an incorrect number of chromosomes, which is a hallmark of many cancers.
Anaphase and Telophase: Separation and Reconstitution
Anaphase is the phase of movement. It occurs in two sub-stages. In anaphase A, the kinetochore microtubules shorten, pulling the sister chromatids toward the poles. In anaphase B, the polar microtubules overlap and slide past each other, elongating the cell. The chromatids are now considered individual chromosomes. Telophase follows, where the chromosomes arrive at the poles and begin to decondense back into chromatin. The nuclear envelopes re-form around the two sets of chromosomes, and the nucleoli reappear. The mitotic spindle breaks down. Although the nucleus has divided, the cell remains a single entity until cytokinesis, which is covered in the next lecture.
The concept
The cell cycle and mitosis
How a cell duplicates and divides its contents faithfully.
A common misconception is that the nuclear envelope breaks down in prophase. In reality, it breaks down during prometaphase. This distinction is crucial because the breakdown allows spindle microtubules to access the kinetochores, a prerequisite for the attachment events that define prometaphase and enable the subsequent alignment in metaphase. C
Worked example
Problem. A diploid cell has a haploid number \( n = 7 \). Calculate the number of centromeres present in the nucleus during metaphase and during anaphase.
- Identify the diploid number \( 2n \) for the cell.
- Determine the number of chromosomes in metaphase, noting that sister chromatids are still joined.
- Determine the number of chromosomes in anaphase, noting that sister chromatids have separated.
- Recall that the number of centromeres equals the number of chromosomes.
Units check. The result is a dimensionless count of structural units, consistent with the integer nature of chromosome numbers.
Three quick tests
- What structure forms at the centromere to attach to spindle microtubules?
Answer
The kinetochore. - During which phase does the nuclear envelope break down?
Answer
Prometaphase. - What is the primary function of the spindle assembly checkpoint?
Answer
To ensure all kinetochores are properly attached to spindle microtubules before anaphase begins.
Back to BU-101 →
Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.